ARROW-6987: [CI] Travis OSX failing to install sdk headers

[mrkn]

macOS_SDK_headers_for_macOS_10.14.pkg is no longer provided in Xcode 11.
We shouldn't depend on it.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-6987

[kou]

macOS job is green: https://travis-ci.org/apache/arrow/jobs/604734693

We can merge this once all CI is green.

[codecov-io]

Codecov Report

Merging #5756 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5756      +/-   ##
==========================================
- Coverage   88.97%   88.96%   -0.02%     
==========================================
  Files         995      995              
  Lines      135328   135328              
  Branches     1501     1501              
==========================================
- Hits       120408   120394      -14     
- Misses      14555    14569      +14     
  Partials      365      365

Impacted Files	Coverage Δ
cpp/src/arrow/filesystem/s3_internal.h	90.74% <0%> (-3.71%)	⬇️
cpp/src/arrow/json/converter.cc	90.05% <0%> (-1.76%)	⬇️
cpp/src/arrow/json/chunked_builder.cc	80% <0%> (-1.67%)	⬇️
cpp/src/arrow/csv/column_builder.cc	95.54% <0%> (-1.49%)	⬇️
cpp/src/arrow/util/task_group_test.cc	98.86% <0%> (-1.14%)	⬇️
python/pyarrow/tests/test_plasma.py	95.76% <0%> (-0.32%)	⬇️
python/pyarrow/tests/test_parquet.py	95.3% <0%> (+0.05%)	⬆️
python/pyarrow/plasma.py	56.16% <0%> (+1.36%)	⬆️

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